WASHINGTON -- President Barack Obama vowed Tuesday to come down like "a ton of bricks" on firms that violate sanctions against Iran, speaking at a joint news conference with French President Francois Hollande, who defended a trip by French businesses executives to Tehran.

The United States and France are among the countries that signed an interim nuclear agreement with Tehran. The agreement halts progress on the Islamic republic's nuclear program in exchange for easing international sanctions. Talks on a final deal begin next week in Vienna, Austria.

Speaking on companies doing business with Iran in violation of sanctions still in place, Obama said, "We will come down on them like a ton of bricks" if they don't hold up their end.

The Obama administration has objected to the interest French businesses have shown in Iran since the sanctions were eased. More than 100 French executives visited Tehran last week, a trip Secretary of State John Kerry told his counterparts in Paris was "not helpful."

Hollande said he told the French businessmen that sanctions remain in effect and no commercial agreements can be signed without a long-term, comprehensive nuclear deal. But he said he's not president of the French employer's union and companies make their own travel decisions.

The United States and France also have been working to end the violent civil war in Syria, a former French colony. But peace talks between the Syrian government and opposition forces have gained no traction.

Obama acknowledged that Syrian peace talks are far from reaching their goal.

"There's enormous frustration here," he said of the negotiations.

An agreement to strip Syria of its chemical weapons stockpiles is being carried out. But there are concerns on both sides of the Atlantic that Syria is stalling on its obligations.

When Obama threatened a military strike against Syria following a chemical weapons attack there last year, France was the only European ally ready to join that effort.

Obama said the United State and France have rebuilt a relationship that "would have been unimaginable even a decade ago," after President George W. Bush launched an unpopular war against Iraq.

There has been some tension between the U.S. and its allies in Europe and elsewhere following revelations that their leaders had been subject to spying from the National Security Agency.

Obama said there is no country with which the United States has "a no-spy agreement." But he says the United States endeavours to protect privacy rights as it collects foreign intelligence.

Hollande said he and Obama "clarified things" about the spying revelations and "mutual trust has been restored."

"That mutual trust must be based on respect for each other's country but also based on protection, protection of private life, of personal data, the fact that any individual, in spite of technological progress, can be sure that he's not being spied on. These are principles that unite us," Hollande said.

Obama also announced that he's accepted Hollande's invitation to travel to France for the June 6 ceremony marking the 70th anniversary of the D-Day invasion of Normandy.

The remarks came at an hour-long news conference in the midst of an official state visit, held as Hollande is facing romantic upheaval that resulted in his showing up stag to the White House. The 59-year-old ended his relationship last month with girlfriend and French first lady Valerie Trierweiler after it was revealed that he was having an affair with an actress.

The White House has carefully avoided any mention of Hollande's personal drama and has moved forward with a grand welcome reserved only for America's closest allies.

On a cold February morning, Obama, first lady Michelle Obama and a military honour guard welcomed Hollande as he arrived on the South Lawn of the White House. The two leaders shook hands before a cheering crowd, many waving American and French flags, and greeted two American military veterans who served in France during World War II.

Following the arrival ceremony, Obama and Hollande held a private meeting in the Oval Office before appearing before the press in the East Room. The Obamas planned to fete Hollande at a grand state dinner Tuesday night attended by more than 300 dignitaries and celebrities.

A French reporter asked if France has now become the top U.S. ally in Europe, beating out Britain. "I have two daughters," Obama replied. "And they are both gorgeous and wonderful, and I would never choose between them. And that's how I feel about my outstanding European partners."